Various techniques for preventing information theft from optical communication systems are known in the prior art. In one such technique, the structure of the transmitting optical fiber is modified either to block access to the core of the fiber to the “writing” radiation necessary to form a grating tap, or to provide additional optical paths that allow monitoring signals to be conducted through the fiber that are separate from the transmission signals conducted through the fiber core. A detected attenuation of the monitoring signals conducted through the fiber indicates a breach in the outer coating or cladding of the fiber. In another technique, a first active device (such as a phase modulator) encrypts the optical signals at a point of transmission, which are later decrypted at a point downstream in the optical network by a second active device.
While either prior art technique is capable of implementing secure optical transmissions, they are not without shortcomings. Techniques that require a modification of the structure of the transmission fiber are not easily applied to existing optical networks, as the existing optical transmission fiber must be replaced. Additionally, such modified fiber would be a relatively expensive “specialty” fiber requiring additional manufacturing steps or and/or modified materials over conventional fiber. Active encryption devices require relatively large amounts of space in the optical network, require an outside power source, and often cannot be easily installed by a simple splice into the transmission fiber. They are also relatively sensitive and failure-prone as compared to passive secure-transmission devices, such as the aforementioned modified fiber.
Clearly, what is needed is a passive device for providing secure optical transmissions that is easily installed on existing optical transmission systems without the need for replacing existing fiber. Ideally, such a device should take up only a small amount of space, provide only a minimum amount of signal attenuation, and should be capable of scrambling an optical signal using a virtually unlimited amount of different “keys” so that it is difficult, if not impossible for information thieves to extract the transmitted information from the scrambled signal.